Office Web Services on SP2010+SP1.   Using Kerberos.  We see PPT and Word rendering fine. Excel gives 503 errors. Cache cleared, services checked, bounced, etc. Nothing showing up in the ULS logs. Apparently this has never worked. Hmm...

After an hour or two of digging away and not seeing anything it began to seem as if we were not getting past IIS. At this point, I found an obscure note and here is what allowed Excel to now render!

Open IIS manager > Open the SharePoint Web Service > Authentication > Disable ASP.NET Impersonation.